The present invention relates to an objective lens composed of a pair of lenses, for use in an optical pick-up for recording or reproducing information on or from an optical information recording medium or the like, and also relates to an optical pick-up device equipped with the objective lens, and an assembling method of the objective lens.
The research and development of the light-utilizing technology have been made in a variety of fields including communication, instrumentation, processing, and the like, for the characteristic of light such as high frequency (high speed), or characteristics which realize spacial information processing capability, phase processing, etc., and such light-utilizing technology has been used in practical applications. In the light-utilizing technology, a high-precision objective lens has been used as means for focusing an optical beam. In recent years, the demand for mass-storage information recording devices such as image recording devices, especially those utilizing light, has been increasing, and a development of technology to meet such demand is therefore strongly desired.
To record mass optical information, it is necessary to improve recording media, and what is more, to form a smaller beam spot, that is, to focus a beam spot sufficiently by an objective lens. As already known, the diameter of a beam spot is proportional to the wavelength of light and inversely proportional to the NA (Numerical Aperture) of an objective lens. As for wavelengths, a blue laser diode and a blue or green SHG (second harmonic generation) laser are being developed. Meanwhile, as for deriving larger NA from objective lenses, the NA of a DVD (Digital Versatile Disc) is 0.6, while the NA of a CD (Compact Disc) is 0.45, showing that high density has been achieved.
However, as far as an objective lens is made up of a single lens with two aspheric surfaces, it is difficult to manufacture an objective lens which provides much larger NA. Therefore, an objective lens and an optical pick-up device in which a 2-group lens (a pair of lenses) is adopted to achieve larger NA are disclosed in Japanese Unexamined Patent Publication No. 10-123410/1998 (Tokukaihei 10-123410, published on May 15, 1998).
FIG. 11 shows an arrangement in which an objective lens made up of such a 2-group lens (a pair of lenses) is assembled and set at a lens barrel. An objective lens 101 includes a light-source-side lens 102, a medium-side lens 103, a first lens barrel and aperture 104, and a second lens barrel 105. The light-source-side lens 102 is held by the first lens barrel and aperture 104, the medium-side lens 103 is held by the second lens barrel 105, and the first lens barrel and aperture 104 and the second lens barrel 105 are fixed together via an adhesive resin layer 106.
In addition, an arrangement to combine a 2-group lens without using a lens barrel is disclosed as a compound lens in Japanese Unexamined Patent Publication No. 10-123388/1998 (Tokukaihei 10-123388, published on May 15, 1998). FIG. 12 shows one example. In a compound lens 111, a light-source-side lens 112 and a medium-side lens 113 are held and fixed by a transparent core member 114 which penetrates the lenses.
In such a 2-group lens as mentioned above, two or more lenses are used and these lenses are held and fixed by a lens holding member such as a barrel-type member, that is, a lens barrel, or a core member, and thus combined.
In the above conventional 2-group lens, it is desirable in terms of rigidity to use metal as a material for a lens holding member, but that results in heavier weight. The use of polymeric resin compounds may then be considered, but since these materials have low rigidity, the proportion of a filler such as glass fiber, glass bead, or carbon fiber is increased to cover the disadvantage, which results in an increase in weight, besides, a rigidity as high as that of metal cannot be obtained.
In addition, in an objective lens using a 2-group lens, as the interval of the two lenses is increased, the length of a lens holding member such as a lens barrel accordingly increases, which causes not only heavier weight but also less rigidity due to a torsion, bend, or the like of the lens holding member.
Furthermore, as for optical characteristics, in the case of the objective lens 101 in FIG. 11, for example, since the light-source-side lens 102 and the medium-side lens 103 cannot be manufactured without a manufacturing error (molding error) in general, it is necessary to adjust the interval between the respective centers of the two lenses 102 and 103 when assembling a 2-group lens to cancel an aberration caused by the above error. Therefore, it is difficult to fix the interval between the centers of the two lenses 102 and 103, in such a manner so as to determine the dimensions of the lens barrel (the first lens barrel and aperture 104 and the second lens barrel 105) beforehand considering the calculated interval between the respective centers of the two lenses 102 and 103, and fix the interval between the center of the lens 102 and the center of the lens 103 uniformly based on the dimensions of the lens barrel. In other words, it is necessary to adopt a method to adjust the positioning of the two lenses 102 and 103 to have an optimal interval between the respective centers of the two lenses 102 and 103, by observing a beam spot, and fix them with resin or the like.
In this method, however, the adhesive resin material 106 of low rigidity is adopted between the first lens barrel and aperture 104 which holds the light-source-side lens 102 and the second lens barrel 105 which holds the medium-side lens 103. Therefore, an overall rigidity of the objective lens 1 is further reduced, thus resonance characteristics when driving an actuator for lens focusing and tracking are deteriorated, making high speed recording and reproducing difficult.
The present invention is achieved in finding a solution to the foregoing problem, and it is therefore an object of the present invention to provide an objective lens including a 2-group lens, which has high rigidity as a whole and is capable of high speed recording and reproducing without deteriorating resonance characteristics when driving an actuator for focus servo and track servo.
In order to attain the above object, an objective lens of the present invention is arranged so as to include a pair of a light-source-side lens provided on the side of a light source and a medium-side lens provided on the side of a recording medium, which are provided facing one another,
wherein at least either one of rim portions of respective opposing surfaces of the light-source-side lens and the medium-side lens forms a rise-up section projected towards the other one of the light-source-side lens and the medium-side lens;
the rim portion which forms the rise-up section and the rim portion of the other one of the light-source-side lens and the medium-side lens at least partially contact one another; and
the light-source-side lens and the medium-side lens are fixed together by an adhesive resin layer formed in a spacing between the rim portions of the respective opposing surfaces.
According to the above structure, at least either one of the rim portions of the respective opposing surfaces of the two lenses forms a rise-up section projected towards the other lens, and further, the rim portion which forms the rise-up section at least partially contacts the rim portion of the other lens, and the light-source-side lens and the medium-side lens are fixed together by an adhesive resin layer formed in a spacing between the rim portions of the respective opposing surfaces.
Therefore, in the objective lens, the light-source-side lens and the medium-side lens, which are made of glass, for example, directly contact one another at their rim portions, and are fixed together by the adhesive resin layer, without using a lens holding member such as a lens barrel.
Therefore, the structure eliminates the weight increase for the use of a lens holding member. In addition, the adhesive resin layer is restrained from serving as an elastic body, thus increasing rigidity. As a result, it becomes possible to raise the resonance frequency or to lessen the amplitude of an actuator which drives the objective lens during focus servo and track servo using the objective lens, thereby achieving high speed operation.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.